1. Field of the Invention
The invention relates to centrifugal pumps and, more particularly, relates to a centrifugal pump having a volute which has improved discharge characteristics when compared to traditional volutes and/or which is easily removable for pump maintenance or cleaning. The invention additionally relates to a method of assembling or disassembling such a pump and to a method of its use.
2. Discussion of the Related Art
Centrifugal pumps, sometimes known as kinetic energy pumps, are widely used in a variety of fluid handling applications. An internal or external power source drives an impeller to draw liquid into a pump chamber and to expel liquid from the pump chamber under pressure. In the most typical configuration, liquid flows into an axial inlet of the impeller, is forced by the impeller through a toroidal flow path formed by a volute surrounding the impeller, and is discharged from the volute and out of the pump.
The volute sometimes needs to be removed from the pump chamber to permit repair or replacement of the volute or the impeller or to permit cleaning of the pump chamber. This need is particularly evident in the case of a so-called xe2x80x9ctrash pump,xe2x80x9d which is configured to pump water or other liquids having a relatively high percentage of comparatively large-diameter solids dispersed in them. Trash pumps are used in a variety of agricultural, construction, and industrial applications. The most common trash pumps have a discharge diameter ranging from two inches to six inches and are configured to pump fluids having solids therein that are half as large as the discharge orifices. Hence, a three-inch trash pump is capable of pumping liquids having entrained solids of a diameter of one and a half inches.
Trash pumps have a tendency to become clogged with foreign matter. Some trash pumps therefore incorporate measures to provide access to the interior of the pump chamber for the purposes of unclogging it. This access is usually provided by way of an access opening that is large enough to permit the volute to be removed from the pump chamber. The access opening is closed during normal operation of the pump by a cover that is mounted on the pump casing by knobs or other user-friendly fasteners that permit the cover to be quickly removed in the field to provide access to the pump chamber interior.
Some volutes are formed integrally with the wall of the pump chambers. Pumps of this design are difficult to clean because there is no way to separate the volute from the pump chamber walls to dislodge materials lodged between them. In order to avoid this problem, the volutes of many trash pumps are removable to facilitate cleaning and maintenance. The volute of the typical trash pump of this type is bolted to the cover so as to permit the cover and the volute to be removed as a unit. However, the volute is attached to the cover from the inside and, therefore, must be removed with the cover. This construction can be problematic when the pump is clogged with materials lodged between the walls of the pump chamber and the volute because the lodged materials tend to jam the volute in place, severely inhibiting its removal from the pump chamber. The operator of a clogged pump may find himself in an unpalatable situation in which the pump is so badly clogged he cannot remove the cover/volute subassembly from the pump to dislodge the clogged materials. In this case, the pump has to be removed from the field for servicing.
Some manufacturers have attempted to solve this problem by not affixing the volute to the cover. The volute instead is simply held in place within the pump chamber by mating surfaces on the cover and the pump casing. This arrangement overcomes the problem of preventing cover removal when the pump is clogged, but creates a new problem of its own. Specifically, if the cover is removed when the volute is not lodged in place with the chamber, the volute may fall out of the pump casing before the operator is ready to deal with it, risking damage to the volute and/or injury to the operator.
The need therefore has arisen to provide ready access to the pump chamber of a trash pump or the like, even if the pump is severely clogged with debris, without fear of dropping the pump""s volute.
Another feature of trash pumps and many other centrifugal pumps is that they are xe2x80x9cself-priming.xe2x80x9d This term is a misnomer because all centrifugal pumps must be primed, that is, at least partially filled with water, before they can operate on their own because they cannot pump air. As a result, unless the pump is located below the water source and water can flow down to the pump chamber by gravity, liquid must be manually poured into the pump chamber before operation. xe2x80x9cSelf-primingxe2x80x9d pumps are those that have an external or internal water reservoir which, when filled with liquid, negates the need for continuous manual priming during periods of low flow. Self-priming capability is enhanced in the typical centrifugal pump through the use of a flapper valve or other one-way valve. The one-way valve prevents flow out of the suction inlet from the pump chamber from the suction inlet and, therefore, assures that the pump chamber will remain full or nearly full during periods of inactivity or low flow.
Self-priming is particularly important in applications in which liquid flow rates into the pump often drop below the minimum discharge rate of the pump. In this instance, when air enters the suction hose for the pump, the pump stops pumping. However, the residual water in the pump chamber primes the pump until the hose is again filled with water. Pumping then resumes and will continue until air again enters the inlet hose.
Trash pumps and many other centrifugal pumps use the so-called xe2x80x9crecirculation methodxe2x80x9d for self-priming. These pumps incorporate a recirculation port in the volute that is too small to feed the impeller all of the water that it is capable of handling. As a result of this and other aspects of the volute""s configuration, air is drawn from the suction opening of the pump by the impeller. The resultant mixture of air and water is repeatedly discharged by the impeller and into the surrounding pump chamber. The water and air bubbles separate in the pump chamber so that the majority of the air is discharged from the pump chamber. The liquid returning to the recirculation port therefore is relatively free of air and increases the density of the fluid flowing through the volute, hence increasing the effectiveness of the pump. The cycle continues until enough air is purged from the suction hose to create a sufficient vacuum to draw water into the pump chamber.
Several factors influence a centrifugal pump""s performance. These factors include its maximum discharge capacity, its maximum total head, and its maximum total suction lift. Volute features that affect maximum discharge capacity and maximum total head are controlled by well-known mathematical calculations. However, volute features that maximize suction lift ability and priming ability are subject to more empirical determinations that have yet to be optimized. Hence, while many self-priming centrifugal pumps use baffles and other relatively convoluted structures within the pump chamber to attempt to enhance the pump""s self-priming capability, there is still considerable room for improvement.
The need therefore has arisen to provide a centrifugal pump volute that improves the pump""s maximum suction lift capability and provides for a higher prime rate.
In accordance with the first aspect of the invention, a volute of a centrifugal pump and an adjacent access cover are fastened to one another and to the associated pump casing so as to permit the cover and the volute to be removed either as a unit or individually, depending upon operator preference and the prevailing circumstances.
Preferably, the fasteners comprise volute fasteners that fasten the cover to the volute and cover fasteners that fasten the cover to the pump casing. Both types of fasteners are accessible from an exterior surface of the cover. The volute fasteners may comprise bolts that extend through bores in the cover and that are threaded into tapped bores in the volute. These fasteners preferably further comprise cover fasteners that are fastened to the pump casing and that are accessible from an exterior of the pump to remove the cover from the pump casing. In order to facilitate field maintenance by minimizing the need to use tools to assemble or disassemble the pump, the cover fasteners preferably 1) cooperate with notches in the cover and 2) can be loosened and swung away from the notches without removing the cover fasteners from the pump casing, thereby permitting removal of the cover from the pump casing. For instance, each cover fastener may include 1) a threaded rod that is attached to the pump casing and 2) a knob threadedly attached to the threaded rod, each of the knobs being generally T-shaped and having 1) a first leg threaded onto the rod and 2) a second leg extending at least generally perpendicularly from the first leg and having a pair of longitudinally-spaced, generally triangular thumb engagement portions.
In accordance with another aspect of the invention, a pump volute includes a discharge that is configured to provide improved maximum suction lift capacity and improved self-priming capability. The pump has a pump casing which has a pump chamber formed therein, a rotatable impeller which is housed in the pump chamber, and a volute housed in the pump chamber. The volute has an inner radial edge which is at least generally circular and which surrounds the impeller, an outer radial surface which is at least general toroidal in shape, a radial outlet in fluid communication with an outlet of the pump chamber, and a recirculation inlet in fluid communication with the radial outlet. The radial outlet is formed from an outlet of a discharge duct. The discharge duct is dimensioned and configured to discharge a liquid stream from the outlet thereof in a flow pattern that is at least substantially tangential to the inner radial peripheral edge.
Preferably, the discharge duct has inner and outer radial walls. The inner radial wall has an outer end portion that extends at least substantially linearly and at least substantially tangentially to the inner radial peripheral edge. The outer radial wall has an outer end portion that extends at least substantially linearly and at least substantially tangentially to the inner radial peripheral edge. The outlet of the discharge duct preferably faces generally upwardly and is inclined downwardly from the inner radial wall of the discharge duct to the outer radial wall of the discharge duct.
In accordance with other aspects of the invention, improved methods are provided of 1) assembly or disassembling a pump so as to provide access to a pumping chamber thereof, even if the chamber is clogged with debris, and/or 2) self-priming a centrifugal pump.